In apparatus for the recovery of ethylene and other unsaturated hydrocarbons, it is common practice to introduce petroleum fractions as the raw material for a pyrolytic cracking process. In recent years efforts have been made to use fractions having a higher boiling point than naphtha for this purpose. The use of, for example, gas oil as the starting material for a pyrolytic cracking process has, however, several technological disadvantages.
With the pyrolysis of naphtha and lighter petroleum fractions, the cracking gas stream from the cracking furnace can be rapidly cooled by indirect heat exchange with production of high-pressure steam to obtain maximum heat utilization and a cool product which does not pose any significant difficulties since undesired subsequent reactions are substantially completely avoided. Further cooling can be carried out by the direct spraying into the cracking gas stream of oil generated in the cracking process.
However, when gas-oil cracking gases are to be cooled from elevated temperatures, various problems are involved. It is not possible, for example, to use indirect cooling methods since the latter are not immediately effective and at the elevated temperatures polymerization products and carbon deposits are formed.
The heat dissipation by indirect cooling is insufficient because of the short operating life of heat exchangers which rapidly become caked with carbonaceous deposits. With heavy petroleum fractions, moreover, insufficient heat transfer can be affected by such indirect process.
Hitherto the heat abstraction has been effected primarily be direct spraying of oil into the cracking gas stream. This also poses problems since, when the cooling oil droplets in the presence of the hot cracking gases contact the duct walls, carbonaceous deposits are formed. It has already been proposed (see German open application -- Offenlegungsschrift No. 2062937) to avoid this problem by introducing the cooling oil into the cracking-gas duct so that it forms a film of cooling oil on the latter duct.
This conventional apparatus for the application of a cooling-oil film to the cracking-gas duct has, however, the disadvantage that the oil film is not always coherent, complete and uniform so that turbulence frequently brings the cooling oil, duct wall and hot cracking-gas stream into contact, carbonaceous deposits being thereby formed.